1. Field of the Invention
This invention relates broadly to systems for receiving Global Positioning System (GPS) satellite signals, and more particularly, to GPS receiving systems employing a phased array communication mechanism.
2. State of the Art
GPS is a satellite navigation system funded by and controlled by the U.S. Department of Defense (DOD). While there are many thousands of civilian users of GPS world-wide, the system was designed for and is operated by the U.S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. The Space Segment of the GPS system consists of a constellation of satellites that send radio signals from space. The nominal GPS Operational Constellation consists of twenty-four satellites that orbit the earth in twelve hours. There are often more than twenty-four operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each twenty-four hours (four minutes earlier each day). There are six orbital planes (with nominally four satellites in each plane), equally spaced (sixty degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight satellites visible from any point on the earth.
Each GPS satellite transmits a microwave signal L1 at a carrier frequency of 1575.42 MHz and a microwave signal L2 at a carrier frequency of 1227.60 MHz. The L1 signal and the L2 signal carry the GPS Navigation Message and binary codes, which shift the phase of the L1 and L2 signals. The binary codes of the L1 signal include a C/A Code (Coarse Acquisition code) and P Code (Precise code). The L2 signal includes only the P Code. The P Code is a repeating 10.23 MHz pseudo-random noise (PRN) sequence. The C/A code is a repeating 1.023 MHz PRN sequence. These noise-like codes provide spread-spectrum modulation of the L1 and L2 signals. The P code is intended for military use and is only available to authorized users. Civilian users access the GPS signals through the C/A code. There is a different C/A code PRN for each GPS satellite. GPS satellites are often identified by their PRN number, the unique identifier for C/A code PRN assigned thereto. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters. The GPS Navigation Message consists of time-tagged data bits. Such time-tagged data bits provided by multiple GPS satellites are used to determine latitude, longitude, height, velocity and the exact time. Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates.
The GPS constellation""s design insures that six to eleven satellites are in view from any point on the earth""s surface at any given time. Because of the GPS signal design, two-dimensional and three-dimensional positions can be determined with the signals from just three and four satellites respectively. Accordingly, GPS receivers typically have the capability of automatically selecting three or four of the satellites in view based upon their received signal strength and Position Dilution of Precision (PDOP).
A number of undesirable interference sources (e.g., deliberate electronic countermeasures, RF electromagnetic pollution, clutter scatter returns and nature noise) can cause a GPS receiver to be ineffective or unreliable. Since GPS signals are very weak, they can be overcome by interference caused by even low power and low cost transmitters. When close to the interference source, GPS receivers are unable to track the satellite signals, and when further from the interference source, GPS receivers are able to track, but not acquire the GPS signals.
Phased array antenna systems have been predominantly used in military and aerospace applications because of their high implementation costs. In such systems, the signals received from a number of antenna elements are supplied to signal processing channels that provide a variable gain and variable phase shift to such signals. An antenna pattern for the combined receive signal can be formed by a set of specific gain values and phase shift values over the signal processing channels and a specific geometry and placement of the N antenna elements. The set of specific gain values and phase shift values is commonly referred to as xe2x80x9cweightsxe2x80x9d (or xe2x80x9cweight vectorxe2x80x9d) for the phased array antenna system. A unique advantage of the phased array antenna system is that the antenna pattern can be adjusted by changing the xe2x80x9cweightsxe2x80x9d as described above to perform one or both of the following operations:
a) beam steering: steering the beam by adjusting the phase shift values of the pattern for each processing channel; no adjustment to the gain values of the pattern is necessary.
b) antenna null: the phase shift values and gain values of the pattern are adjusted to the suppress signal (i.e., interference) from a specific direction.
U.S. Pat. No. 6,246,369 to Brown et al. describes a phased array antenna system for use in conjunction with a GPS receiver. The phased array antenna system nulls interference sources and/or applies gain through beam steering in the direction of the desired GPS signal sources. FIGS. 1A and 1B are block diagrams that illustrate this phased array antenna system, which includes a digital front end (DFE) unit 70, a digital beam steering (DBS) card 71 and a receiver processor board 72 that reside inside a personal computer 74 and that are controlled by a software program through the computer data bus. A plurality of DFE channels 63 within the DFE unit 70 convert the analog signals output from the antenna elements 18 to a digital sampled signal. Each of the DFE channels 63 operates from a common reference local oscillator (REF LO) 61 and a common sample clock 64 which is synchronized to the local oscillator 61. The outputs from the plurality of DFE channels 63 are passed to the DBS card 71 where the digital phase shifting is applied. The DBS card utilizes digital signal processing logic blocks 62 to apply complex weights to the input digital signals and form a digital summation to thereby provide composite complex digital output signals to a plurality of channels 73 of the receiver processor board 72.
The DSP logic blocks 62 operate under control of the personal computer 74 to provide the complex weights to adjust the antenna array pattern in order to track the GPS satellites as they move across the sky, to apply calibration corrections to compensate for offset between the individual antennas and the DFEs, or to apply nulling in the direction of the an interference source.
FIG. 1B illustrates the circuit components for each DFE channel 63 of the DFE unit 70, which operates to down-convert the GPS signals from radio frequency (RF) to intermediate frequency (IF) and to sample and convert the analog IF signal into a digital data stream. The GPS signals received at the antenna element 81 are passed through a low-noise amplifier 82, a ceramic filter 83 and another amplifier 84 for output to a mixer 85. The mixer 85 mixes this signal with coherent signals generated by a common local oscillator 61. The mixed and down-converted signals are then passed through a surface acoustic wave (SAW) filter 86 to form the IF signals. The IF signals are then passed through an amplifier 87 and an automatic gain control stage 88, which is operated under control of computer 74 to set the correct levels for analog-to-digital converter 89. The output of the A/D converter 89 is a sampled digital data stream that represents the digitized GPS data signals from each antenna element.
One of the drawbacks of this architecture is that it utilizes a large number of analog-to-digital converters 89 (e.g., one for each antenna element), which substantially increases the cost of the system. Another drawback is that the input signal level of the analog-to-digital converter 89 needs to be at a substantially higher level as compared to that of the received signal at the antenna element 81. Thus, the received signal needs to be amplified by one or more stages of amplifiers in order to bring the received signal to a level that the analog-to-digital converter 89 can operate properly. Such multistage amplification increases the cost of the system. Yet another drawback of this architecture is that is difficult to maintain the signal delays precisely through the DFE channels because the number of processing elements between the antenna element 81 and the analog-to-digital converter 89 is high. Such precise signal delays are required for accurate beam steering and nulling operations. Thus, calibration of these signal delays is required, which limits the suitability of this prior art phased array antenna system in many GPS applications that require limited maintenance by the end user.
GPS receivers also suffer from multipath errors which are caused by the receiver receiving a composite of the direct GPS signals and reflected GPS signals from nearby objects such as the ground or nearby buildings. Occurrence of such multipath errors is common within cities with high rise buildingsxe2x80x94often referred to as an xe2x80x9curban canyonxe2x80x9d. Automobile manufacturers and their suppliers are one of the largest consumers of GPS receivers for in-vehicle navigation systemsxe2x80x94and their use of such systems continues to grow. A typical GPS antenna can receive-both direct line of sight (LOS) signals and multipath signals. The direct line of sight (LOS) signal and the multipath signal are summed according to their relative phase and strength, resulting in a composite signal which has a timing epoch differing from that of the direct line of sight (LOS) signals. The GPS receiver is incapable of distinguishing and xe2x80x9crejectingxe2x80x9d the reflected signal from the direct line of sight (LOS) signal unless the signal propagation delays of the two signals differ by at least half a chip duration (or 150,000 meters). In the xe2x80x9curban canyonxe2x80x9d, the direct line of sight (LOS) signal could be completely blocked by a building structure and a reflected signal could reach the receiver, resulting in a position and velocity error of significant magnitudes. The magnitude of the error depends in part on how far away the structure causing the signal reflection is.
In a paper by Brown entitled, xe2x80x9cMultipath Rejection Through Spatial Processing,xe2x80x9d Proceedings of ION GSP 2000, Salt Lake City, Utah, September, 2000, the sampled digital data stream that is output by the digital-to-analog converter 89 of each DFE channel 64 of the phased array antenna of FIGS. 1A and 1B is processed to dynamically estimate the presence of multipath signal sources. Beam steering operations are used to provide antenna gain to reinforce direct signals and nulling operations are used to minimize the effect of received multipath signals. A maximum likelihood algorithm is used to implement the beam steering/multipath minimization operations on the digitized GPS data signals from each antenna element.
The drawbacks of the architecture of this phased array antenna system as described above also limit its suitability in addressing multipath rejection. More specifically, precise signal delays are required for accurate beam steering/nulling operations that are required for effective multipath rejection. Thus, calibration of these signal delays is required, which limits the suitability of this prior art phased array antenna system in many GPS applications that require limited maintenance by the end user. In addition, the maximum likelihood algorithm operates to select maximum signal power components, In some cases, such as when the direct path signal is blocked by a building, this may give erroneous results.
Thus, there remains a need in the art for improved phased array antenna systems for use in conjunction with a GPS receiver that are cost effective; that provide precise phase delay through the processing channels of the system for accurate beam steering/null operations that are required for effective multipath rejection; and that are suitable for applications that require limited maintenance by the end user.
It is therefore an object of the invention to provide an improved GPS antenna/signal receiver that employs a phased array antenna with precise phase delay through the processing channels of the antenna for accurate beam steering with minimal costs.
It is a further object of the invention to provide an improved GPS antenna/signal receiver that employs a phased array antenna with precise phase delay through the processing channels of the antenna and that is suitable for applications that require limited maintenance by the end user.
It is an additional object of the invention to provide an improved GPS antenna/signal receiver that employs a phased array antenna with a minimal number of signal processing elements in each processing channel prior to signal combination.
It is still another object of the invention to provide an improved GPS antenna/signal receiver that employs a phased array antenna with precise phase delay through the processing channels of the system for accurate beam steering utilizing simple and cost effective components.
In accord with these objects, which will be discussed in detail below, an improved GPS signal-receiver (and corresponding method of operation) includes a plurality of antenna elements each receiving, a plurality of GPS signals (e.g., GPS LI signals or GPS L2 signals). A plurality of mixers, which correspond to the array of antenna elements, cooperate with a combining node to convert the GPS signals received at the antenna elements in a frequency-division-multiplexed (FDM) manner over FDM frequency bands logically assigned to the antenna elements, to thereby produce a composite signal representing such GPS signals. An analog-to-digital converter converts an: analog signal derived from the combined signal (which may be an IF signal or baseband signal) into a digital word stream. Demultiplexing logic extracts GPS signal components in the digital word stream. The GPS signal components correspond to the FDM frequency bands logically assigned to the antenna elements. Beam forming logic, operably coupled to the demultiplexing logic, applies variable phase delay and variable gain to each GPS component in accordance with a set of weight values supplied thereto.
According to one embodiment of the present invention, the improved GPS signal receiver: (and corresponding method of operation) includes beam forming control logic that analyzes GPS signal components extracted by the demultiplexing logic to identify a given set of weight values that nulls at least one interfering signal, and that dynamically supplies the given set of weight values to the beam forming logic to null the interfering signal(s). Preferably, the given set of weight values are derived by identifying the direction of arrival for one or more interfering signals and generating the given set of weight values such that antennae nulls are provided for directions corresponding to the direction of arrival for the interfering signal(s). Advantageously, such processing provides for improved interference cancellation and multipath rejection.
In another embodiment of the present invention, the improved GPS signal receiver (and corresponding method of operation) includes beam forming control logic that estimates a pointing direction of a GPS satellite with respect to the GPS signal receiver and dynamically adjusts the beam steering direction (i.e., set of weight values supplied to the beam forming logic) in accordance with the estimated pointing direction. Preferably, the estimated pointing direction is based upon satellite position data provided by a GPS receiver, generated by scanning mode operations (that scan over a range of antenna pointing, directions to identify a pointing direction with maximum signal strength), or generated by tracking mode operations (that dither over a range of antenna pointing directions to identify a pointing direction with maximum signal strength). Such signal processing operations improve the overall signal to noise ratio of the GPS signals received by the system.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.